The invention relates to a supercharging air compressor for an internal combustion engine, to an internal combustion engine and to a method for that purpose.
The document DE 198 08 633 A1 discloses compressing sucked-in combustion air for an internal combustion engine to supercharging pressure in a supercharging air compressor, the temperature of which air is reduced in a supercharging air cooler which is connected downstream. So that the supercharging air temperature is additionally reduced, a cooling air compressor is provided in parallel with the supercharging air compressor in the intake system, a cooling air cooler and a diffuser, in which the cooling air is relaxed to ambient pressure, being connected downstream of said supercharging air compressor. The cooling air is then fed to an additional cooler in the intake tract in which the supercharging air which emerges from the supercharging air cooler is additionally cooled once more. This additionally cooled supercharging air is then fed to the cylinders of the internal combustion engine.
It is possible to reduce the temperature of the supercharging air using this structurally complex system. However, it remains problematical that, in particular when high supercharging pressures are required, the temperatures increase greatly directly at the compressor outlet of the supercharging air compressor, which leads to high thermal loading of the components of the supercharging air compressor. In order to ensure that the compressor functions according to the specifications, it is necessary either to use high quality materials which withstand these thermal loads, which is however associated with considerable material costs, or to cool the compressor housing, which is associated with high additional structural complexity and a result of which the thermal loads of the compressor wheel are not reduced, or not reduced significantly.
The invention is based on the problem of reducing the thermal loading in a supercharging air cooler using simple measures.
This problem is solved according to the invention with a supercharging air compressor having the following features.
In the supercharging air compressor according to the invention, a partial stream of the compressed supercharging air is branched off downstream of the compressor wheel, is led through a temperature reducing unit which is assigned to the supercharging air compressor, and is subsequently fed to a component of the supercharging air compressor, in particular to the compressor wheel, as cooling air. In this way, the temperature level of the components of the supercharging air compressor is reduced, specifically in particular the temperature of the compressor wheel and also the temperature of the compressor housing. It is also possible to use, for example, cost-effective aluminium as the material for the compressor housing and the compressor wheel, even at high supercharging pressures. The fed-back cooling air stream ensures sufficient cooling of the supercharging air compressor.
It may be expedient to adopt an embodiment in which the magnitude of the fed-back partial air stream which is used for cooling can be adjusted as a function of the state variables and operating variables of the internal combustion engine. The fed-back partial stream can in particular also be completely shut off if this is permitted by the temperature conditions in the supercharging air compressor. On the other hand, the fed-back cooling air stream is limited only by the magnitude of the mass stream passing through the supercharging air compressor.
The feeding back of the partial air stream with temperature reduction constitutes a cost-effective measure for cooling the supercharging air compressor which is easy to implement. The temperature reducing unit may be embodied in particular as a passive component, in particular as a diffuser, which consumes no additional energy. In such a diffuser, the temperature reduction is brought about by relaxing the fed-back air mass stream. The relaxing is advantageously carried out to ambient pressure, at which the fed-back cooling air is preferably introduced into the compressor inlet duct, in which the fed-back mass stream is combined with the combustion air which is sucked in from the surroundings.
The temperature reducing unit is advantageously connected to the compressor housing and forms, for example, a single-piece component with it. This embodiment is defined by a particularly compact design.
After the partial stream has flowed through the temperature reducing unit, the cooled air is introduced in particular into the compressor inlet duct, as a result of which the compressor wheel and the walls which surround the compressor wheel are cooled. The cooled air can be introduced in particular via an additional duct which opens into the compressor inlet duct adjacent to the compressor wheel, preferably directly upstream of the axial end side of the compressor wheel blade and/or radially at the level of the compressor wheel blade. This additional duct may be part of what is referred to as a characteristic-diagram-stabilizing measure which can be used to extend the operating range of the compressor. As a result, the additional duct can axially bridge the compressor wheel inlet edge and form a bypass to the compressor inlet duct, as a result of which the pumping limit of the compressor can be displaced in favour of smaller mass streams. When the compressor is operating near to the pumping limit, the bypass permits a partial mass stream to be recirculated counter to the main flow direction in the compressor inlet duct. The fed-back partial mass stream is fed back to the main mass stream and conveyed back into the compressor via the bypass. The pumping limit is displaced here in favour of a larger operating range. Such a bypass or additional duct can then also be used for introducing the cooling air stream, the additionally introduced mass stream also influencing the pumping limit positively.
In the internal combustion engine according to the invention having a supercharging air compressor in the intake tract, a temperature reducing unit is provided downstream of the compressor wheel, it being possible to feed a partial stream of the compressed supercharging air to said temperature reducing unit. The partial stream is fed, as cooling air, to the supercharging air compressor after it has flowed through the temperature reducing unit. The partial stream is branched off here in particular downstream of a supercharging air cooler in the intake tract and fed back to the supercharging air compressor. In this way, two-stage cooling can be implemented for the fed-back partial air stream; firstly in the supercharging air cooler and then in the temperature reducing unit.
In the method according to the invention for cooling the supercharging air compressor, a partial stream of the supercharging air which is compressed in the supercharging air compressor is branched off downstream of the compressor wheel, the temperature of this partial stream is reduced and finally the partial stream is fed back, as cooling air, to the supercharging air compressor. The magnitude of the fed-back partial air stream can expediently be adjusted here; it is in particular possible to shut off the partial mass stream completely.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.